Vitamin D insufficiency is deemed as an unrecognized epidemic among healthy adults and adolescents residing in the United States. Epidemiologic and clinical data clearly document the high prevalence of vitamin D insufficiency among adults and adolescents. However very little is known regarding the prevalence, seasonal variation and the metabolic impact of vitamin D insufficiency among pre- and early adolescent childhood populations. Using a longitudinal study design, we will estimate the prevalence and seasonal variation of vitamin D insufficiency in 6 to 12 year old pre- and early adolescent children and characterize its metabolic impact on their skeletal health. On the basis of the calculated sample size estimate, a cohort of 140 healthy pre- and early adolescent children (6-12 year old, Tanner I or II, African American [n=94] and Caucasian [n=46]) will be evaluated during summer (June through September) and winter (December through March). Vitamin D status, calcitrophic hormones, and markers of bone turnover will be assessed during summer and winter. Other measures collected will include dietary intakes of vitamin D and calcium, sun exposure and body mass index. Vitamin D insufficiency will be defined as serum 25 hydroxyvitamin D < 20 ng/mL However, on the basis of the relationship between serum 25 hydroxyvitamin D and PTH, the threshold level of serum 25 hydroxyvitamin D essential for vitamin D sufficiency will be determined. This study will establish the prevalence, seasonal variation, and the metabolic impact of vitamin D insufficiency in young school age African American and Caucasian children. The public health importance of childhood vitamin D insufficiency is linked to the impact of vitamin D status on the acquisition of peak bone mass. Reduced peak bone mass will predispose to premature onset of osteoporosis and osteoporotic fragility fractures. Treatment and prevention of vitamin D insufficiency during childhood will positively impact the skeletal health of children and reduce the "osteoporosis burden" during adulthood. The paucity of data regarding threshold vitamin D levels associated with vitamin D sufficiency status among young school age children and the potential likelihood that the threshold serum 25 hydroxyvitamin D levels essential for vitamin D sufficiency could be different between African American and Caucasian children, makes it compelling for this issue to be explored. [unreadable] [unreadable] [unreadable]